Perhaps if you told us a little more you might get some help...
1. What system do you have, Windows, Mac, Linux?
2. Have you got the Lego software installed and working?
3. Have you read the tutorials for leJOS?
4. hat have you tried so far?

As I said...
2. Do you have the original Lego software installed and working on your PC?
4. What have you tried so far?

If you have the original Lego software installed and working on both your PC and nxt, then you should be able to just install leJOS. This download new software to your nxt, so you will lose any programs you may already have stored on it. You should not need to reset the nxt if you have the Lego software installed on it.

I had similar problem when I have stared with NXT, so when I was done I have published everything online. You can find this information on this page:
http://nxt.tiim.info/ or download everything from here: http://www.tiim.info/the_nxt_project/web/download.phpStart with Installation and Setup Guide (iCommand, Eclipse and BlueCove)
Hope it helps you any questions just ask.